René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"assert.deepEqual([null], [undefined]) behavior changed in v22.16.0 (undocumented breaking change)","articleBody":"### Version\n\nv22.16.0+ (and v24.x)\n\n### Platform\n\nAll platforms\n\n### Subsystem\n\nassert\n\n### What steps will reproduce the bug?\n\n```javascript\nconst assert = require('assert');\nassert.deepEqual([null], [undefined]);\n```\n\n### How often does it reproduce? Is there a required condition?\n\nAlways reproduces on Node \u003e= 22.16.0. Passes on Node \u003c= 22.15.0.\n\n### What is the expected behavior? Why is that the expected behavior?\n\nThe assertion should **pass** because:\n\n1. The [documentation](https://nodejs.org/api/assert.html#assertdeepequalactual-expected-message) states: \"Primitive values are compared with the `==` operator\"\n2. In JavaScript, `null == undefined` is `true`\n3. This worked in all Node versions prior to 22.16.0\n\n### What do you see instead?\n\n```\nAssertionError [ERR_ASSERTION]: Expected values to be loosely deep-equal:\n\n[ null ]\n\nshould loosely deep-equal\n\n[ undefined ]\n```\n\n### Root Cause\n\nPR #57619 (labeled as \"performance improvement\") introduced this change. The modification in [`objEquiv`](https://github.com/nodejs/node/pull/57619/files#diff-2da3538f1eacd02ed1b3c8f7fe3f8c0cee640295830a6f0dd4a6205326578958R785-R793) now checks `a[i] !== undefined` with **strict equality** (`===`) before calling `innerDeepEqual`, which would have used loose equality (`==`):\n\n```javascript\n// Before: called innerDeepEqual() which respects loose equality mode\nif (!innerDeepEqual(a[i], b[i], mode, memos)) { return false; }\n\n// After: checks with strict equality BEFORE calling innerDeepEqual\nif (b[i] === undefined) {\n  if (a[i] !== undefined || !hasOwn(a, i))  // \u003c-- uses === not ==\n    return false;\n}\n```\n\nWhen comparing `[null]` vs `[undefined]`:\n- `b[0]` is `undefined` → enters first branch  \n- `a[0] !== undefined` → `null !== undefined` is `true` (strict equality)\n- Returns `false` immediately, never calls `innerDeepEqual` which would use `==`\n\n### Additional information\n\n- This is an **undocumented breaking change** in an LTS release\n- The PR was not labeled as semver-major\n- Affects anyone comparing arrays that may contain `null` values against expected `undefined` values (common with MongoDB/BSON which stores `undefined` as `null`)\n\nI understand `deepEqual` is marked as \"Legacy\" and docs warn it \"can have surprising results\", but this specific change contradicts the documented behavior and broke existing code without notice.","author":{"url":"https://github.com/iholovin","@type":"Person","name":"iholovin"},"datePublished":"2026-01-30T08:04:02.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/61583/node/issues/61583"}